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Science & Global Security, 1989, Volume I, pp.93-107Photoropying pennitted by license onlyReprinte available directly from the publisherC> 1989 Gordon and Breach Science Publishers SAPrinted in the United States of America

Background on SpaceNuclear Power

Steven Aftergooda

This paper introduces the technology of space nuclear power, reviews the historyof its deployment, provides background information on current development pro-grams, and examines the proposed applications of space nuclear power systems.

SPACE POWER CONCEPTUAL DESIGN SUMMARY

A space nuclear power system converts the energy from a nuclear heatsource into electricity to power a particular load or application. The ele-ments of this process may be outlined as follows:

ENERGY STORAGEmechanical,chemlcal

i J,--"" HEAT-TO-ELECTRICHEAT SOURCE -, POWER CONVERSION ~ POWER CONDITIONING ~ LOAD

nuclear reactor t Ii dyno .or /sot sacor mlC

ope open or closed cycle

J,HEAT REJECTION

radiator

a. Executive Director, Committee to Bridge the Gap, 1637 Butler Avenue, Los Angeles,CA 90025

This paper was written under the auspices of the Cooperative Research Project on ArmsReductions, a Joint project of the Federation of American Scientists and the Committeeof Soviet Scientists for Peace and Against the Nuclear Threat

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94 Aftergood

Two basic types of nuclear power supply have been used in space-nuclear reactors and radioisotope sources. In a space nuclear reactorsystem, the energy source is the heat generated by the controlled fissionof uranium. This heat is transferred by a heat-exchange coolant to eithera static (for example, thermoelectric) or dynamic (for example, turbine/alt-ernator) conversion system, which transforms it into electricity. Thiselectricity can then be "conditioned" into the form needed by the payload.Waste heat is rejected through a radiator.

In an isotope power supply, the heat is produced by the natural decayof a radioisotope, which in all US-launched systems is plutonium-238.1This heat, like that produced by reactors, can be converted to electricityby a static converter or by a dynamic conversion system. And again, thiselectricity is conditioned to meet payload requirements, and waste heat isradiated away.

Nuclear power supplies offer significant reductions in mass, comparedwith the alternatives, when power needs exceed several tens of kilowattsfor more than several days. Quantitative estimates for the chemical andsolar power equivalents to the SP-100 and multi-megawatt space nuclearreactors are listed in table 1. The general applicability of various energysources, nuclear and non-nuclear, for a range of power and durationrequirements is illustrated in figure 1. For all practical purposes, nuclearreactors are required when moderate to high levels of continuous powerare required for an extended period.

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DURATION Of USE

Figure 1: Regimes of Possible Space Power ApplicabilitySource: DoE

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Background on Space Nuclear Power 95-

Table 1: Solar and Chemical Power Equivalen1s of Two Space Reactors

Nominal Chemical Solar panel Solar panelprojected equivalent equivalent equivalent

mass mass mass areakg kg kg m'

(1 kWhr/kg) (lOW/kg) (130W/m2)

SP-l00 -5.000 -6 x 1r1' 10.000 750700 kWe. 7 years

Multi-megawatt -50.000 -9 x 107 1 x 1r1' 75.00370 MWe. 7 year

HISTORY OF SPACE NUCLEAR POWER

US ProgramsThe US began to develop small nuclear power sources for use in space in1955 as part of the SNAP (Systems for Nuclear Auxiliary Power) pro-

gram.The US has launched a total of 22 spacecraft powered by one or more

radioisotope thermal generators (RTGs). In addition, the US has deployedone reactor-powered satellite. A list of these space nuclear power systemsis presented in table 2. The sources used in these missions were all quitelow power. The most powerful, the SNAP lOA reactor, generated only 500watts of electricity.

The US space reactor program was shelved in 1973, because no mis-sions then required a space reactor. It was not revived until the start ofthe SP-100 program, described below.

The most recent US RTG-powered spacecraft was launched in 1977.NASA's Galileo voyage to Jupiter is the next RTG-powered mission andas of early 1989 was scheduled for launch in October 1989.

Soviet ProgramsThe Soviet Union has launched over 30 nuclear reactor powered satellitesand several RTG-powered satellites and lunar modules. A list of nuclear

powered spacecraft launched by the USSR is presented in table 3.

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Space reactors are used by the Soviet Union to power their RadarOcean Reconnaissance Satellites (RORSATs), which track and target US

naval vessels. The altitude of a RORSAT orbit is typically between 270

and 255 kilometers. This low orbit, which enhances the satellite's radar

capability, also dictates the use of nuclear power rather than solar panels,

since the latter would increase drag and significantly shorten the lifetime

of the orbit. In addition, solar-powered spacecraft would require an electri-

cal storage system for operation in the Earth's shadow, adding mass and

technical complexity.

At the end of a RORSAT's mission of typically two to three months,

the reactor is designed to separate from the satellite and be boosted to a

Table 2: Space Nuclear Power Systems Launched by the US

launch date Spacecraft Power Mean Status/source altitude lifetime

km29 Jun 1961 Transit 4A RTG- 930 shut down15 Nov 1961 Transit 4B RTG 1.030 nonoperational28 Sep 1963 Translt-5BN-1 RTG 1,095 9 months5 Dec 1963 Transit-5BN-2 RTG 1,085 norioperational21 Apr 1964 Transit-5BN-3 RTG aborted3 Apr 1965 Snapshot reactor 1 ,290 43 days

18 May 1968 Nimbus-B-1 RTG aborted14 Apr 1969 Nimbus III RTG 1,100 nonoperational14 Nov 1969 Apollo 12 RTG on lunar surface11 Apr 1970 Apollo 13 RTG aborted31 Jan 1971 Apollo 14 RTG on lunar surface26 Jul 1971 Apollo 15 RTG on lunar surface2 Mar 1972 Pioneer 10 RTG beyond Pluto16 Apr 1972 Apollo 16 RTG on lunar surface2 Sep 1972 Transit-01-1X RTG 770 RTG operating7 Dec 1972 Apollo 17 RTG on lunar surface5 Apr 1973 Pioneer 11 RTG beyond Saturn

20 Aug 1975 Viking 1 RTG on Mars9 Sep 1975 Viking 2 RTG on Mars

14 Mar 1976 LES 8 RTG 35,785 RTGs operating14 Mar 1976 LES 9 RTG 35,785 RTGs operating

20 Aug 1977 Voyager 2 RTG beyond Uranus5 Sep 1977 Voyager 1 RTG beyond Saturn

.All us RTGs are fueled by plutonlum-238; the Snapshot reactor was fueled by uranlum-235.

Sources: Gary L. Bennett, James J. Lombardo, and Bernard J. Rock, "Development and Use of NuclearPower Sources for Space AppllcatkJns: Journal of fhe Astronautical ScIences. 29, 4, October-Decem-ber 1981, pp.321-342; Nk::hoias L. Johnson, "Nuclear Power Supplies In Orbit: $pace Palicy, August1986, pp.223-233. Mean a~ltude given as of 1 January 1986.

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Background on Space Noc/ear Power 97~

Table 3: Nuclear Powered Spacecraft Launched by the USSR

launch date Spacecraft Power Mean lifetimesource altitude

km3 Sep 1965 Cosmos 84 RTG' 1,50018 Sep 1965 Cosmos 90 RTG 1,500

27 Dec 1967 Cosmos 198 reactor 920 1 day22 Mar 1968 Cosmos 209 reactor 905 1 day25 Jan 1969 possible RORSAT launch failure23 Sep 1969 Cosmos 300 RTG re-entered22 Oct 1969 Cosmos 305 RTG re-entered

3 Oct 1970 Cosmos 367 reactor 970 1 day1 Apr 1971 Cosmos 402 reactor 990 1 day

25 Dec 1971 Cosmos 469 reactor 980 9 days21 Aug 1972 Cosmos 516 reactor 975 32 days25 Apr 1973 RORSAT launch failure

27 Dec 1973 Cosmos 626 reactor 945 45 days15 May 1974 Cosmos 651 reactor 920 71 days17 May 1974 Cosmos 654 reactor 965 74 days

2 Apr 1975 Cosmos 723 reactor 930 43 days7 Apr 1975 Cosmos 724 reactor 900 65 days

12Dec 1975 Cosmos 785 reactor 955 1 day17 Oct 1976 Cosmos 860 reactor 960 24 days21 Oct 1976 Cosmos 861 reactor 960 60 days16 Sep 1977 Cosmos 952 reactor 950 21 days18 Sep 1977 Cosmos 954 reactor re-entered -43 days29 Apr 1980 Cosmos 1176 reactor 920 134 days5 Mar 1981 Cosmos 1249 reactor 940 105 days

21 Apr 1981 Cosmos 1266 reactor 930 8 days24 Aug 1981 Cosmos 1299 reactor 945 12 days14 May 1982 Cosmos 1365 reactor 930 135 days

1 Jun 1982 Cosmos 1372 reactor 945 70 days30 Aug 1982 Cosmos 1402 reactor re-entered 120 days

2 Oct 1982 Cosmos 1412 reactor 945 39 days29 Jun 1984 Cosmos 1579 reactor 945 90 days31 Oct 1984 Cosmos 1607 reactor 950 93 days

1 Aug 1985 Cosmos 1670 reactor 950 83 days23 Aug 1985 Cosmos 1677 reactor 940 60 days21 Mar 1986 Cosmos 1736 reactor 950 92 days20 Aug 1986 Cosmos 1771 reactor 950 56 days

1 Feb 1987 Cosmos 1818 reactor 800 -6 months18 Jun 1987 Cosmos 1860 reactor 950 40 days10 Jul 1987 Cosmos 1867 reactor 800 -1 year

12 Dec 1987 Cosmos 1900 reactor 720 -124 days14 Mar 1988 Cosmos 1932 reactor 965 66 days

.The RTGs are believed to be fueled by pok>nium-210, whk:h has a half-life of 13839 days.

Source: Nk:halas L. Johnson, "Nuclear Power Supplies In Orbit; Space Policy, August 1986, pp.227, 228.Revised and updated by personal communication with Johnson, 24 June 1988. Mean altitude is givenas of 1 January 1986 for pre-1986 launches. Cosmos 1818 and Cosmos 1867 are believed to be flighttests of a new Topaz-type reactor.

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98 Aftergood

higher, 1ong-1ived orbit of about 950 ki1ometers. Booster fai1ure resu1ted inthe premature re-entry of the reactor aboard Cosmos 954 in 1978, whichscattered radioactive debris over northwest Canada.

Following that incident, the system was redesigned to expe1 the reac-tor core from the reactor at the end of the mission to faci1itate disintegra-tion in the high atmosphere in the event of re-entry. This was the casewhen Cosmos 1402 re-entered in 1983. Even when boosting is successfu1,the reactor core is still ejected once the higher orbit is attained.

Further design changes were disc10sed in 1988 that inc1uded automat-ed boost systems.2 These are triggered by one of three fai1ures: a 10ss ofattitude contr01, reactor depressurization, or disruptions in the e1ectrica1system. The first of these apparent1y triggered the 1ast-minute boost ofCosmos 1900 in 1988.

Pub1ished detai1s of Soviet space nuc1ear power programs are sparseand sometimes contradictory.! In 1988, however, the Soviets issued infor-mation on the Cosmos 1900 reactor:4

The reactor's core consists of 37 cylindrical heat-releasing elements withfacing beryllium reflectors. A uranium-molybdenum alloy with 90 percenturanium-235 enrichment is used as nuclear fuel (total weight 31.1 kilo-grams) A1so, judging from an ana1ysis of the Cosmos 954 debris, these Soviet

space reactors operate on fast neutrons.Soviet officia1s recent1y disc10sed that two new "Topaz" reactors were

flight-tested in 1987-88: The tests were performed at a power 1eve1 of 10kilowatts e1ectric at orbits of about 800 kilometers a1titude and with anoperating time of 6 months and 1 year respectively. The satellites bearingthe Topaz reactors have been tentative1y identified by Western observersas Cosmos 1818 and Cosmos 1867.6

SPACE NUCLEAR POWER ACCIDENTS AND FAILURES

A remarkably large fraction-about 15 percent-of all US and Sovietnuc1ear powered space missions have ended in accidents, 1aunch aborts, orother failures. These incidents are briefly described be10w in chronologica1order:7

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Background on Space Nuclear Power 99

.1964When the US 1ransit-5BN-3 navigational satellite failed to achieve orbiton 21 April, its SNAP 9A RTG power source disintegrated in the atmo-sphere (as it was designed to do in case of re-entry) at an altitude ofabout 50 kilometers. Release of its 17,000 curies of plutonium-238 tripledthe worldwide environmental inventory of plutonium-238 and increasedthe total world environmental burden (measured in curies) from all pluto-nium isotopes (mostly fallout from atmospheric nuclear weapons testing)by about 4 percent.8

.1968On 18 May, the US Nimbus-B-l meteorological satellite was abortedfollowing a launch failure, and fell into the Pacific Ocean just off theCalifornia coast. Five months later, its two SNAP 19A RTGs were retriev-ed intact.

.1969A Soviet launch failure occurred on 25 January that may have involved anuclear powered RORSAT.

.1969On 23 September and 22 October the USSR launched unmanned probesto the moon. Both achieved earth orbit, but re-entered the atmosphere afew days later. According to various sources, one or both of them carried Ia polonium-210 heat source. Measurable amounts of radioactivity were ,detected in the atmosphere following re-entry.9

.1970A US moon mission, Apollo 13, was aborted in April. Its jettisoned lunarlander fell into the Pacific Ocean. The SNAP 27 plutonium power supplyhas never been recovered, but atmospheric sampling detected no release ofradioactivity, and the RTG is assumed to have remained intact.

.1973On 25 April a Soviet nuclear powered RORSAT fell into the Pacific Oceanafter a launch failure.

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i .1978

In one of the most serious accidents involving space nuclear power, theSoviet Cosmos 954 re-entered the atmosphere on 24 January, spreadingthousands of pieces of radioactive debris over more than 100,000 squarekilometers of northwest Canada. 10 A few fragments were highly radio-

active (gamma radiation as high as 500 roentgens per hour near contact).

.1983

The jettisoned reactor core from Cosmos 1402 re-entered the atmosphereon 7 February, where it disintegrated and was dispersed.11

.1988

Radio contact with Cosmos 1900 was lost in April, preventing a directedboost of the satellite to a high-altitude disposal orbit. Backup systemswere finally activated on September 30, just days before anticipated re-entry, and the on-board reactor was boosted to a higher orbit. Ii

CURRENT US SPACE NUCLEAR POWER PROGRAMS

SP-100The US has several space nuclear power dev,elopment programs underway. The SP-100 reactor is the cornerstone of this effort. Its design is stillunder revision, but many of the important design parameters have beendetermined, at least tentatively, and these are presented in table 4. TheSP-100 System Configuration is illustrated in figure 2. A more detailedview of the reactor itself is portrayed in figure 3. The gamma and neutronradiation profiles of a preliminary SP-100 design are summarized in table5.

The proposed SP-100 has a power level of 2.3 megawatts thermal,which is converted thermoelectrically into 100 kilowatts electric. Themass-to-power objective for the reactor is 30 kg/kWe. The actual estimatedweight of the reference design is about 4,600 kilograms, or 46 kg/kWe.The reactor subsystem, including reactor vessel and shielding, is quitesmall, occupying roughly a cubic meter in volume. The SP-100 as a whole,however, is considerably less compact, with an overall length of about 25meters and a radiator panel surface area of about 100 square meters.

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Background on Space NI.K:/ear Power 1 01

Table 4: Selected SP-l00 Design Parameters

Thermal power 2.3 megawattsElectric power 100 kilowattsOperational lifetime 7 years full power over a la-year

periodFuel uranium nitrideFuel mass -190 kilogramsFuel enrichment 89-97 percent uranium-235Energy conversion thermoelectricRadiator area 106 square metersRadiator temperature -800 kelvinsReactor vessel diameter 35 centimetersNeutron shield lithium hydrideGamma shield tungsten jTotal shield mass 1,000 kilograms !

;,Based on proceedings of the SP-l00 Project Integratk:>n Meeting. 19-21 Jut)' 1988, long Beach, iCalifornk!. I

iTable 5: SP-l00 Radiation Profile ~'1

Core Unshlelded Shield Payloadcenter reactor surface surface"

surface (-23 meters)

Fast neutrons -1 x lQ23 -3 x 1021 -3 X 1015 4 X 1012(n/cm2 over 7 years)

Fast neutron flux 5 x 1014 -1.3 x 1013 -1.3 x 1 Q7 2 x 1 Q4 .(n/cm2-sec)

,,Gamma -1 x 1013 -5 X lOll -2 x loa 5 x lOS ,.

(rods over 7 yrs)

Gamma 4.5 x 1Q4 -2.3 x lQ3 -9 x 10-1 2.3 x 1~(rads/sec)

Neutron attenuation by shield: 7.5 x lOS

Gamma attenuation factor by tungsten: 70by lithium hydride: 50

Total gamma attenuation by shield: 3.5 x lQ3

.These estimates V1clude neutron and gamma attenootion due to the separatk>n distance nvolvedas weN as to shielding. Based on General Electric's SP-IOO Ground Engineering System Baseline DesignStudy, 1984, pp.3-28.

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Figure 2: SP-1OO Deployed Configuration

Source: Jet Propulsion Lab

IN CORESAFETY

REACTOR VESSEL RODS

HINGED REFLECTOR FUEL BUNDLESCONTROL SEGMENT & HONEYCOMB

REENTRY STRUCTUREHEAT SHIELD THAW ASSIST

HEAT PIPESREACTOR SHIELD-

PRIMARY HEATTRANSPORT

SUPPORT PIPING ANDSTRUCTURE INSULATIONSTRUTS POWER

CONVERTERREACTOR I&C AUXILIARYMULTIPLEXER COOLING

LOOP GASAUXILIARY SEPARATORCOOLING LOOP ACCUMULATORRADIATOR AUXILIARY

INCORE COOLING LOOPTEM PUMPSAFETY ROD- AND STARTER

ACTUATOR RADIATOR

INTEGRATION STRUCTURALJOINTS INTERFACE

) RING

Figure 3: SP-1OO Reactor Power Assembly

Source: Jet Propulsion Lab

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Background on Space Nuclear Power 103-

The SP-100 design concept could be downscaled to about 10 kilowattselectric or upscaled to about 1 megawatt electric. Above 1 megawattelectric it would be unacceptably heavy; therefore this concept is not

suitable for multi-megawatt applications.

Multi-megawatt ReactorsThe US Department of Energy is also investigating the feasibility ofdeveloping space nuclear reactors with power levels above 1 megawattelectric in its Multimegawatt (MMW) Program. Budget limitations willprobably push design concept selection beyond 1991, and finaldevelopment of a MMW reactor beyond the year 2000,

More than a dozen MMW system concepts have been proposed, butmany major feasibility issues remain to be resolved. The Department ofEnergy has subdivided the multi-megawatt reactor requirements into

three categories:13

.Tens of megawatts, open cycle (working fluid vented)

.Tens of megawatts, closed cycle

.Hundreds of megawatts for hundreds of seconds, open or closed cycle.

Isotope Power SystemsThe isotope power systems are divided into radioisotope thermal genera-tors (RTGs), which use thermoelectric conversion, and dynamic isotopepower systems (DIPS), which employ dynamic energy conversion. Bothsystems use plutonium-238 as the radioisotope heat source.

RTGs have evolved over the last 30 or so years from the early SNAPsystems up to today's General Purpose Heat Source (GPHS), which isintended to be used aboard NASA's Galileo (two RTGs) and Ulysses (oneRTG) missions. The GPHS will have a thermal power of 4.4 kilowatts andwill contain a total plutonium mass of 9.4 kilograms!'

Radioisotope heat sources are also used in small quantities to providethermal energy to critical spacecraft components that might be adverselyaffected by low temperatures. The Galileo mission to Jupiter would useabout 130 such Light-Weight Radioisotope Heater Units (LWRHU) (1 wattthermal per unit) in addition to its two GPHS RTGS.15

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104 Aftergood

A DIPS would provide electric power in the range 1-10 kilowattselectric. Above about 1 kilowatt, RTGs are no longer acceptably low inmass. Above 10 kilowatts, the mass of a DIPS power supply becomesexcessive, and a small nuclear reactor becomes the preferred option.

DIPS proponents point out that DIPS has a lower infrared signature(less heat radiated: see below), lower radar signature (it is compact), anda lower nuclear signature (neutron or gamma emission) compared withfission reactors.IS

However, a single 6-kWe DIPS system would require a rather stun-ning 53 kilograms of plutonium-238.17 This is about two and a half timesthe amount of fallout of all plutonium isotopes (measured in curies) fromall atmospheric nuclear weapons tests.

MILITARY APPLICATIONS OF SPACE NUCLEAR POWER'8

The applications of current and proposed space nuclear power supplies,particularly in earth orbit, are predominantly military.

As noted above, the Soviet Union uses nuclear reactors to powerreconnaissance satellites that track and target US naval vessels.19

In the US, the various space nuclear power programs are driven bythe Strategic Defense Initiative, with its development of high-poweredspace weapons and associated orbiting platforms. Without SDI, therewould be little US demand in the near term for most of the space nuclearpower supplies now being developed.

Because the architecture of a Strategic Defense System is far frombeing finally determined, it is impossible to define the precise role ofnuclear power in the ultimate system. But there is a broad consensusamong the Strategic Defense Initiative Organization, the Department ofEnergy, the American Physical Society (APS) Study Group on DirectedEnergy Weapons, and the Office of Technology Assessment that nuclearpower reactors in space are likely to be an essential component of thelater phases of SDI-those that would employ directed energy weapons.

Members of the APS Panel explained that even20

a few tens of kilowatts of electrical power necessitates nuclear power reactorsfor two reasons. First is survivability: The large area needed for solar cellswould make a satellite very vulnerable to actions of the offense. Second is

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Background on Space Nuclear Power 1 05

reliability: The long expected stay in orbit could reduce the availability ofpower [from solar cells] because of the radiation damage that occurs over 10-year time scales 8m officials have divided their power needs into three categories:

housekeeping (or baseload), alert mode, and burst mode. Housekeepingwould require from several up to about 100 kilowatts electric of continu-ous power for various maintenance and control functions, such as surveil-lance, communication, data processing, attitude control, and refrigeration,throughout the life of the system. The alert mode would be engaged at atime of impending conflict and, according to the 8m Organization, couldrequire as much as 10 megawatts of electricity for up to a total of per-haps one year over the entire mission lifetime. The burst mode, involvingthe actual pulsing of a directed energy weapon, could require hundreds ofmegawatts or more for tens to hundreds of seconds. Preliminary estimatesof power requirements for specific 8m systems are presented in table 6.

Table 6: Estimated Average Power Requirements for Space Assets kW

Mode of operation Base Alert BurstBoost suNeiliance and

tracking satellite 4- 1 0 4- 10 4-10

Space suNelliance andtracking satellite 5-15 5-15 15-50

Laser radar (Ladar) 15-20 15-20 50-100

Ladar imager 15-20 15-20 100-500

Laser Illumination 5-10 5- 1 0 50-100Doppler ladar 15-20 15-20 300-600

Space-based interceptor carrier 2-30 4-50 10-100

Chemical laser 50-100 100-150 100-200

Fighting mirror 10-50 10-50 20-100

Neutral particle beam/Space-based free electron laser 20-120 1,000-10,000 1 x lOS-5 x 1~

Electromagnetic launcher (rallgun) 20-120 1,000-10,000 2 x lOS-5 x lr:1'

Source: Strategic Defense Initiative Organization. Reprinted n SDI: Technology. Surv/vabRity , a1dSoftware (Washington DC: US Congress. Office of Technology Assessment. OTA-ISC-353. May 1988).p.142.

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NOTES AND REFERENCES

1. Production-grade plutonium-238 actually consists of about 83 percent pluto-nium-238, 15 percent plutonium-239, and small amounts of other plutoniumisotopes. The half-life of plutonium-238 is 87.8 years.

2. See the testimony of Daniel Hirsch in "Cosmos 1900 and the Future of SpaceNuclear Power," hearings before the Committee on Energy and Natural Resources,United States Senate, 13 September 1988. See also T.M. Foley, "Soviet Nuclear-Powered Satellite Expected to Reenter Within 30 Days," Aviation Week & SpaceTechnology, 19 September 1988, p.20.

3. R. Townsend Reese and Charles P. Vick, "Soviet Nuclear Powered Satellites,"Journal of the British Interplanetary Society, 36, 1983, ppA57-462, is interesting,though it contains inconsistencies. Nicholas L. Johnson, "Nuclear Power Suppliesin Orbit," Space Policy, August 1986, pp.223-233, is extremely informative, as isJohnson's annual Soviet Year in Space (Colorado Springs, Colorado: Teledyne

~~ Brown Engineering).

4. Report by the USSR State Committee for the Use of Atomic Energy, submittedto the International Atomic Energy Agency, reprinted in a telegram from the USMission in Vienna, Austria, to the US Secretary of State, 27 September 1988.

5. William J. Broad, "Russians Disclose Satellites Carry New Reactor Type," NewYork Times, 15 January 1989, p.A1; "Nuclear Power Plant for Space VehiclesDeveloped," Tass News Agency, 5 January 1989.

6. Personal communication, Nicholas Johnson, 24 June 1988.

7. Further information and source citations may be found in S. Aftergood, "Nucle-ar Space Mishaps and Star Wars," Bulletin of the Atomic Scientists, October 1986,ppA0-43.

8. E.P. Hardy, P.W. Krey, and H.L. Volchok, "Global Inventory and Distributionof Fallout Plutonium," Nature, 16 February 1973, pA44.

9. Johnson, 1986, p.227; William J. Broad, "Satellite's Fuel Core Falls 'Harmles-sly'," New York Times, 8 February 1983, pp.Cl,C2.

10. W.K. Gummer et al., Cosmos 954: The Occurrence and Nature of RecoveredDebris, (Hull, Quebec: Canadian Government Publishing Center, INFO-0006, May1980), p.11.

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Background on Space Nuclear Power 107'--'--' 11. Robert Leifer et al., "Detection of Uranium from Cosmos-1402 in the Strato-

sphere," Science, 238, 23 October 1987, pp.512-3.

12. William J. Broad, "Soviet Nuclear Powered Satellite Expected to Hit Earth inSummer," New York Times, 14 May 1988; William Harwood, "Crippled SovietSatellite Fires Reactor into 'Graveyard' Orbit," Washington Post (UPI), 2 October1988, p.A6.

13. Philip J. Klass, "Technical, Political Concerns Impede Space Nuclear Power,"Aviation Week & Space Technology, 1 February 1988, p.59.

14. Gary L. Bennett et al., ~e General Purpose Heat Source RadioisotopeThermoelectric Generator: Power for the Galileo and Ulysses Missions," Proceed-ings of the 21st lntersociety Energy Conversion Engineering Conference, volume 3,August 1986, pp.1999-2011.

15. Light-Weight Radioisotope Heater Unit Safety Analysis Report, ReportMLM-3293, October 1985.

16. Gary L. Bennett and James J. Lombardo, "Technology Development of Dyna-mic Isotope Power Systems for Space Applications," Proceedings of the 22ndlntersociety Energy Conversion Engineering Conference, volume 1, August 1987,pp.366-372. DIPS has recently been redesignated as TECS (Turbine EnergyConversion System).

17. Assuming 20 percent conversion efficiency, and 0.56 Wth/gram plutonium-238.

18. For further information on applications, see S. Aftergood, "Towards a Ban onNuclear Power in Earth o;bit," Space Policy, February 1989, pp.25-40.

19. Nicholas L. Johnson, The Soviet Year in Space 1985 (Colorado Springs,Colorado: Teledyne Brown Engineering, 1985), p.39; see also US Department ofDefense, Soviet Military Power 1985, p.53; and Aviation Week & Space Technology,26 August 1985, p.23.

20. "Debate on APS Directed-Energy Weapons Study," Physics Today, November1987, pp.52-53.

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